THE STEM AND THE LEAF 51 



arrival of cold weather, but it occurs just as certainly, and 

 often after a briefer period of growth, in regions where there 

 is no cold winter. 



48. Annual thickening. In stems such as those of dicotyle- 

 donous trees and the trees of the Pine family and other cone 

 bearers, which live for many years, the cambium forms a new 

 layer of bark and of wood every year. 1 These annual layers are 

 usually more noticeable in the wood than in the bark, because 

 the wood cylinders thus formed remain closely joined together 

 (Fig. 35). The newer lighter-colored portions of the wood are 

 known as sapwood, and the older portions, often darkened by 

 the deposit of coloring matter, are known as heartwood. Not 

 infrequently the heartwood decays and leaves the tree hollow. 



(1) How old is the stick of wood shown in Fig. 35 ? (2) 

 Did it grow equally fast during each year of its life ? Dis- 

 cuss this question. (3) Why is the name " annual rings " not 

 an accurate one ? What are they really ? (4) Is each year's 

 growth uniform all round the stem? (5) Had this stem any 

 branches in the portion shown by the section ? How could 

 the age of the stem, at the time when a branch began, be 

 known (Fig. 37)? 



The hardwood trees show great differences in the rate at 

 which their trunks increase in thickness. Poplars, bass woods, 

 willows, or red oaks, growing in good soil and unshaded, may 

 for forty or fifty years form annual rings as much as three 

 eighths of an inch thick. But old beeches and sugar maples 

 in the forest, after they have passed the hundred-year limit, 

 often grow not more than about one sixteenth of an inch per 

 year. When very old, though still sound, they may grow only 

 about one twenty-fifth of an inch per year. 



Two of the most important of our coniferous or needle- 

 leaved timber trees are the white pine and the long-leaf 

 pine. A white-pine tree, overtopping most of its fellows in 

 the forest, is, on the average, at ten years 0.9 inch in diameter, 



1 In the tropical regions, where there is no marked change of seasons, the 

 wood often grows rather evenly all the year round. 



